The present invention relates to data transmission systems and, more particularly, to the removal of crosstalk interference in a communication system.
Digital subscriber line (DSL) technology uses existing telephone twisted pairs to provide high-speed internet access services to both residential and business customers. There are many types of DSLs, which are generically referred to as xDSL, including basic rate DSL (ISDN), high-bit-rate DSL (HDSL), second generation HDSL (HDSL2), asymmetric DSL (ADSL), symmetrical DSL (SDSL), and very-high-bit-rate DSL (VDSL). Information regarding this area can be found in Chen, “The Development and Standardization of Asymmetrical Digital Subscriber Line”, IEEE Communications Magazine, Volume 37, Number 5, pp. 68-72, May 1999. Today in the United States, several million telephone lines between central offices (COs) and subscribers are deployed with xDSL technology, and the number of the subscribers is rising rapidly.
Multicarrier modulation is a means of transmitting data by first dividing the binary digital data stream to be transmitted into blocks or frames. The blocks are then further grouped into sub-blocks for assignment to a given transmission symbol. As an example, each sub-block may contain one to eight bits, and 100 sub-blocks will form a frame. For analog transmission across the communication channel, a set of carrier signals is used with frequencies that lie at evenly spaced values across the usable frequency band. This set of carrier signals is equal in number to the number of sub-blocks within a block. Continuing the above example, with a sampling frequency of 1.0 MHz, 100 carrier signals would be used at multiples of 10 kHz. The bits within each sub-block are then used to modulate the corresponding carrier signal. The set of modulated carriers are then added together, and the resultant signal is transmitted across the channel. Multicarrier modulation is described more fully in “Multicarrier Modulation for Data Transmission: An Idea whose Time Has Come”, J. A. C. Bingham, IEEE Communications Magazine, 28(5):5-14, May 1990.
In particular, ADSL can use discrete multitone transmission (DMT) as its line code. DMT is a form of multicarrier modulation that is implemented with digital signal processing. Frequently, the method of modulation in the transmitter uses an Inverse Fast Fourier Transform (IFFT), which generates samples of a transmit signal for each block of data bits. In the receiver, the received signal is demodulated in connection with each of the carriers, and the data bits are recovered from each carrier. A complementary or reciprocal method of demodulation samples the received signal, group the samples into blocks, and perform a Fast Fourier Transform (FFT). DMT is discussed in detail in “Discrete Multiple Tone Modulation with Coset Coding for the Spectrally Shaped Channel”, A. Ruiz, J. M. Cioffi, and S. Kasturia, IEEE Transactions on Communications, 40:1012-29, June 1992.
A wide band modulation approach like DMT can present obstacles. One particular problem relates to crosstalk interference that is introduced to the twisted pair transmission line and received by the DSL modem. As is well known to those skilled in the art, crosstalk interference is unwanted interference (signal noise) that is passed between adjacent lines, network cables or other devices. Crosstalk generally occurs due to coupling between wire pairs when wire pairs in the same or a nearby bundle are used for signal transmission. In this manner, data signals from one or more sources may be superimposed on and contaminate a data signal from a different source. Crosstalk can include near-end crosstalk (NEXT) and far-end crosstalk (FEXT). FIGS. 1A and 1B of the drawings illustrate these two types of crosstalk.
In FIGS. 1A and 1B, there are two DSL lines or channels, DSL1 170(1) and DSL2 170 (2), and crosstalk 130, 160 that leaks from DSL1 to DSL2. In FIG. 1A, NEXT crosstalk 130 contaminates the received signal of DSL2 from the transmit signal of DSL1, where the transmitter 110(1) of DSL1 is located near the receiver 110(2) of DSL2. In FIG. 1B, where the transmitter 140(1) of DSL1 is located far from the receiver 150(2) of DSL2, FEXT crosstalk 160 from the transmit signal of DSL1 contaminates the received signal of DSL2 through lines 170(1) and 170(2).
In Japan TCM (Time Compressed Multiplex) ISDN is used, which is a potential source of crosstalk to a DSL system. For that reason, ADSL may have two additional specifications. One of them is ADSL Annex C. The other is ADSL Annex H, which is also named SSDSL (Synchronized Symmetrical DSL) and is synchronized with ISDN to avoid NEXT between it. SSDSL is a synchronized, time division duplexed (TDD) system and, as used herein, the terms “SSDSL” and “synchronized TDD DSL” generally refer to the same type of DSL system.
A brief description of ADSL Annex C is given in, K. Narumiya, “A Consideration of ADSL Service under NTT's Network”, IEEE Communications Magazine, Volume 37, Number 5, pp. 98-101, May 1999. Both Annex C and SSDSL are synchronized with TCM ISDN with a 400 Hz period, which uses a half-duplex transmission method. However, there is a difference in the frequency band between them. Annex C uses the band from 25 kHz to 138 kHz for upstream and from 138 kHz to 1.104 MHz (552 kHz) for downstream. On the other hand, SSDSL uses the band from 25 kHz to 1.104 MHz for both upstream and downstream. There are many systems in which FEXT interference may arise. However, it has not been a big problem for them, because the amount of FEXT is small enough not to affect received signals. On the other hand, there are a few systems in which NEXT interference is included among DSLs, because systems have been designed to avoid NEXT interference that would be big enough to affect received signals. However, for example, upstream signals of SSDSL and downstream signals of VDSL use the same frequency band, so NEXT may arise among them.
NEXT interference in these synchronized, time division duplexed systems is explained in detail in U.S. Pat. No. 5,887,032, entitled METHOD AND APPARATUS FOR CROSSTALK CANCELLATION, and issued Mar. 23, 1999, which is incorporated herein by reference in its entirety. NEXT cancellation in a synchronized TDD system also is described in detail in the '032 Patent.
As can be appreciated, data signals being transmitted over twisted-pair phone lines can be significantly degraded by the crosstalk interference generated on one or more adjacent twisted-pair phone lines in the same and/or a nearby bundle. Therefore, crosstalk problems arising from using twisted-pair phone lines with high data transmission rates, including ADSL and VDSL for example, can substantially inhibit proper transmission of data signals. Thus, there is a need to provide apparatus, methods, techniques and/or computer program products to jointly remove and/or reduce crosstalk interference effects.